The current production of heavy oil involves the use of large amounts of high-pressure steam injected in the geological zones where the heavy oil is embedded (for example in Steam-Assisted Gravity Drainage—SAGD). During the extraction process, the temperature in the steam-injection zone is increased, causing a reduction of the heavy oil viscosity. The heavy oil then drains towards a collector from which it is pumped to the surface, where it is recovered for in situ upgrading or transportation to an upgrader.
Approximately, the enthalpy contained in 1 barrel (bbl) of oil needs to be consumed to produce the steam in order to lift 3 to 4 bbl of heavy oil. The cost of the heavy oil extraction is thus significantly dependent on the cost of the steam produced.
Steam for such operations may be produced by combustion of petroleum coke, vacuum bottoms, heavy oil itself or its asphaltenic fractions once separated from the oil. However, the presence of sulfur, nitrogen and metals (V and Ni for example) in these feedstocks require extensive treatment of the large amount of flue gases generated during combustion to lower particulate and pollutant emissions to below regulatory levels. The presence of vanadium may also be a problem for the refractory present in the boilers used for the combustion which typically operate at high temperatures. Indeed, in an oxidative medium, V2O5 tends to form, which compound has a melting point of 690° C., and to readily create deposits on the refractory walls of combustion boilers, which causes operational problems with time on stream. In addition, combustion does not permit an easy and economical recovery of CO2.
The use of natural gas constitutes an alternate approach to generate the required steam: gas-fired boilers are compact devices and less expensive than boilers for the feedstocks previously specified. However, natural gas pricing is subject to market fluctuations which inevitably influence the heavy oil extraction costs.
An economic alternative to natural gas itself is gasification. There is extensive literature on gasification processes. Gasification processes in petroleum refinery can generally be classified in three broad categories with regard to the gasifier used, namely:
(a) Fixed bed (also called moving bed) gasification;
(b) Bubbling fluid bed gasification; and
(c) Entrained/circulating bed gasification.
With respect to the gasification of refinery residues, the entrained bed gasifiers are usually considered the gasifiers of choice. Well-known examples of such commercial gasifiers include those by Texaco, Dow (E-gas process) or Shell (Higman, C. and van der Burgt, M. (2003). Gasification. Burlington, Mass., Gulf Professional Publishing, an Elsevier imprint., pp 109-128). They involve high temperatures in the reaction zone, approaching 1500° C., to ensure high gasification rates resulting in at least 98% carbon conversion. The high temperatures attained in these gasifiers make them suitable for the gasification of less reactive feedstocks, such as petcoke. However, such high temperatures also imply rather high operational costs and require large scale-ups (about 100,000 bbl per day and more) to absorb the costs.
A conventional fluid bed configuration derived from Winkler's initial low severity fluid bed design (Higman, C. and van der Burgt, M. (2003), Id. pp 101-104) was originally designed for coal gasification. Such low severity configuration has not been considered satisfactory for carbonaceous matrices such as petcoke due to the low reactivity of the carbon structures present in petcoke. The requirements for higher severities have led to a higher severity version of the Winkler design (often referred to as the High Temperature Winkler gasifier, although the most noticeable development has been the increase of pressure) and, ultimately, to more complex circulating beds and entrained bed configurations.
It has been desired for quite a long time in the oil industry that gasification of heavy petroleum residue-derived feedstocks, which generally have a rather high sulfur content, be performed under low severity conditions, that is to say below about 1000° C. and below about 10 atm, while guarantying a balance between reasonable operational costs and commercially satisfying conversion rates. However, such a gasification process still has to be developed.
Therefore remains a need for a low-cost method to produce a synthetic gas from a low value, poorly reactive feedstock consisting of sulfur-containing heavy petroleum residues.
The present description refers to a number of documents, the content of which is herein incorporated by reference in their entirety.